Time (sec)
3.6 Summary
Table 3.5:Comparison of system variables with other studies. All values reported at 1σlevel unless otherwise stated.
Reference Precision (%) Accuracy (%) Limit of Detection (ppt)
Wevill and Carpenter (2004) ∼8 ±15 0.05
O’Brien et al. (2009) 3.2 2.2 0.4
This study 12.7 ±15 0.73
Zhou et al. (2008) 7.5 0.48±0.04
Yokouchi et al. (2005) <20 Jones et al. (2011) 6.51 (2σ)
Figure 3.25:Calibration curve of ECD system from permeation oven. Adapted from Kuyper et al., 2012.
accuracy of the system may be substantially affect by these factors. This may be of particular concern under certain circumstances such as mixing ratios close to the LOD, large peak overlap and unstable peak areas. During these encounters the error of measurement may result in large errors (Plass-Dülmer et al., 2002).
3.6. Summary 65 operation and maintenance of the PerkinElmer F-22 ECD ensured that running repairs could be made and that a relatively unsophisticated initial skill set was required. Helium carrier gas was passed through the DB-624 capillary column at a flow rate of 5 mlmin−1 to achieve the separation of samples. The carrier gas was complemented with nitrogen make-up gas flow through the ECD at a ratio of 1:6. The manual injection valve used to direct gas flow through the TDU trap and to the column was simple to maintain and offered suitable injections. Precise injection volumes might have improved the chromatography further; however the cryo-focusing used resulted in limited band broadening of eluting peaks. Sample pre-concentration by means of an adsorbent bed within the TDU was used in this system. Samples were trapped from the air in the TDU adsorbent bed at a core temperature of -10 ◦C and flow rate of 100 mlmin−1. The trapped samples were desorbed upon injection by heating of the TDU to 300◦C by means of an internal resistance coil. Air samples were pushed through the adsorbent bed by a piston air pump and a mass flow controller regulated the sample gas flow rate through the trap. A chemical desiccant, magnesium perchlorate, was used to dry air samples prior to trapping on the adsorbent bed.
Moisture trapped on the adsorbent bed may possibly damage the column upon injection and lead to an increase in breakthrough from the trap. A breakthrough volume of 5lwas determined for this system. Samples were trapped on the adsorbent bed for 15 minutes resulting in a 1.5ltrapped sample volume. Bromoform should not have broken through the trap, base on these smaller sample volumes used. Calibration of the system was achieved by the introduction of external standards from a permeation oven or sparging flask. Calibration samples from either source were trapped on the adsorbent bed. The contents of a constantly swept 100µlsample loop were repeatedly flushed onto the adsorbent bed for analysis. This resulted in mixing ratios between 5 and 90 ppt being analysed for calibration. The overall precision of the method was found to be 12.7 % with a limit of detection of 0.73±0.09 ppt. While slightly elevated compared to previous studies, this is similar and therefore suitable for the quantitative detection of bromoform in air samples. Bromoform mixing ratios reported at Cape Point (Chapter 4) range between 2.29 – 84.7 ppt. The LOD below 1 ppt suggests that the GC system developed for this study was satisfactorily sensitive to quantitatively detect bromoform mixing ratios from environmental air samples.
A two-stage injection procedure was used to achieve high quality chromatography, with little band broadening and co-elution of compounds. Trapped samples were thermally desorbed from the adsorbent bed and re-trapped on a ‘freezeout-loop’ at the head of the column. Liquid nitrogen, used to achieve the trapping, was replaced by boiling water to desorb the sample for separation and analysis. The detected response from the ECD was recorded on a custom-built MATLAB GUI. The analogue output from the ECD was converted into a digital signal and read by the computer at rate of 4 iterations a second. Repeated injections of clean calibration slugs suggested that the retention time of bromoform in this system was approximately 14 minutes. Bromoform was identified in chromatograms based on retention time. A trapezium peak area integration method was used. The manually identified bromoform peak inflection points were used in the MATALBTRAPZfunction. The method and system described here have been used in the subsequent chapters for bromoform measurements of air samples at Cape Point and water
samples obtained in a laboratory culture experiment.
Several of the components developed for the GC-ECD system described here are consistent with previously published reports, however they have been simplified here to reduce cost (Wevill and Carpenter, 2004; Moore and Groszko, 1999). The result of integration of these components is an instrument with the analytical capability to detect bromoform at ambient (ppt) environmental concentrations in the southern African region where there is currently neither a long-term dataset nor the availability of funding or skills to place a pre-existing instrument. The novelty of the system arises not from the individual components so much as the novel opportunity to contribute to science in a data-poor region. The long-term measurement of bromoform in the atmosphere or ocean has not been performed in southern Africa. This locally developed system and method provides the capacity for the routine quantitative detection of bromoform mixing ratios in environmental air samples along with possible culture studies.
Chapter 4
Bromoform measurements at Cape Point, South Africa
This study examines the character and possible mechanisms of variability of bromoform mixing ratios in the marine boundary layer, as measured at Cape Point. Measurements were performed in the austral spring of 2011. Bromoform mixing ratios were contrasted against local meteorological conditions at the time.
Bromoform mixing ratios at Cape Point were found between 2.27 to 84.7 ppt with a mean of 24.7 ppt.
The wind direction, drawing air from varying sources, appears to have been the dominant mechanism resulting in the observed bromoform variability.